BEIRUT — Fighters from a shadowy group with suspected links to al-Qaeda joined Syrian rebels in seizing a government missile defence base in northern Syria on Friday, according to activists and amateur video.

It was unclear if the rebels were able to hold the base after the attack, and analysts questioned whether they would be able to make use of any of the missiles they may have spirited away.

Nevertheless, the assault underscored fears of advanced weaponry falling into the hands of extremists playing an increasingly large role in Syria’s civil war.

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Videos purportedly shot inside the air defence base and posted online stated that the extremist group, Jabhat al-Nusra, participated in the overnight battle near the village of al-Taaneh, five kilometres east of the country’s largest city, Aleppo. The videos show dozens of fighters inside the base near a radar tower, along with rows of large missiles, some on the backs of trucks.

A report by a correspondent with the Arabic satellite network Al-Jazeera who visited the base Friday said Jabhat al-Nusra took the lead in the attack, killing three guards and taking others prisoner before seizing the base. The report showed a number of missiles and charred buildings, as well as fighters wearing black masks.

Two Aleppo-based activists and Rami Abdul-Rahman, head of the Syrian Observatory for Human Rights, also said Jabhat al-Nusra fought in the battle with other rebel groups. They disputed the notion that the extremist group had the lead role in the attack, however.

Despite Western opposition to President Bashar Assad’s regime, the U.S. and other countries have cited the presence of extremists among the rebels as a reason not to supply the Syrian insurgents with weapons. They have repeatedly cited concerns of heavy weaponry falling into wrong hands.

The base’s capture also plays into fears about extremists acquiring Syria’s chemical and biological weapons — particularly if the Assad regime collapses and loses control of them.

Little is known about Jabhat al-Nusra, or the Support Front, which began claiming attacks in Syria earlier this year in postings on jihadi forums often used by al-Qaeda.

Meanwhile, the fallout deepened from a Syrian passenger jet forced to land in neighbouring Turkey, as Russia said the plane travelling from Moscow to Damascus was carrying radar parts that were being transported legally.

Russian Foreign Minister Sergey Lavrov said the plane was carrying a legitimate cargo of “electric equipment for radars,” but he added it was of “dual purpose,” meaning it could have civilian and military applications.

Turkey’s prime minister has said the plane was carrying ammunition and military equipment for the Syrian Defence Ministry.